A desalination apparatus having a reverse osmosis membrane (hereinafter, referred to as a “RO membrane”) has a plurality of RO membrane elements 222 which are arranged in series inside a pressure vessel 224 of a cylindrical shape to use reverse osmosis pressure, as shown in FIG. 12, and each RO membrane element 222 is connected to a water collection pipe 234 which is located in the center of the RO membrane elements 222. Feedwater is supplied from one side of the desalination apparatus by a high pressure pump, and the inside of the pressure vessel 224 is pressurized in accordance with an open degree of a valve provided on the side of concentrated water. When the applied pressure exceeds the osmotic pressure of the feedwater, desalinated water (permeated water) flows into the central water collection pipe 234 through the RO membranes.
The feedwater supplied into the pressure vessel 224 has higher salt concentration as the feedwater moves toward the concentrated water side from the feedwater side, so that the pressure applied to the pressure vessel 224 can be eventually determined by a salt concentration, an amount of the permeated water, and a flow rate of the feedwater on the membrane surface in the final stage. Therefore, the amount of the permeated water increases on the feedwater side in the pressure vessel 224, because the pressure more than necessary is applied thereto. For example, FIG. 13 shows a relationship between “element position” of the RO membrane and “relative flux”, when seven RO membrane elements 222 are arranged in series. As shown in FIG. 13, the permeated water volume decreases as the element position moves from the feedwater side to the concentrated water side. This is because the salt concentration of an untreated water increases as the element position moves from the feedwater side to the concentrated water side, and a pressure higher than the pressure on the feedwater side is required on the concentrated water side. However, the pressure applied on the feedwater side is the same with that on the concentrated water side, so that more permeated water can be produced on the feedwater side than on the concentrated water side. Since the amount of the permeated water is not uniform in the pressure vessel 224 as shown in FIG. 13, a required power is increased and the RO membrane elements on the feedwater side is contaminated.
To solve such a problem, for example, a seawater desalination apparatus which includes a plug for blocking the water collection pipe on a connection portion of the RO membrane element in the middle of the pressure vessel, and permeated water lines through which the permeated water separated back and forth by the plug is respectively discharged to the outside of the pressure vessel, is described in Patent Document 1. It is further described that the seawater desalination apparatus has an energy recovery apparatus which regulates an amount of front-side permeated water separated by the plug in the pressure vessel to recover the back pressure.